Well, it's not a cheater tag, but a "we suspect you used a hack" thing.
Of course I didn't do that, because I really don't care about achievement, excluding for some games I completed (but I completed only around 20 games in seven years, most of them are stupid games like Finding Teddy).
Jshackles said me a the suspect tag triggers when you unlock a lot of achievements in one session, at the same moments.
This happened me a lot with the original Binding of Isaac, since achievements were bugged and, after you did them, you need to delete the flash.exe file, reboot the game and they unlock at the same time.
Same thing for Fez: I had the GOG version before the Steam one, I fully completed the game here (I posted also a complete savegame on GOG), then while I booted Steam version, the game unlocked all achievements in the same moment.
Anyway, it's just stupid that the site consider legit only 45 achievements over more 4500 I have :l